I don't need the shot because I'm positive. It's only an issue if I'm negative and the baby is positive -- positive can accept negative, in pregnancy or blood transfusions, but negative cannot accept positive.

A friend told us her first child's jaundice (not colic) was probably due to her being negative and the baby being positive. She more or less expected her second child to be admitted back to the hospital a few days after birth for the same reason. (Check the linked wiki article, under "Extrinsic causes of hemolysis", for "Rh disease".)